A federal magistrate in Florida yesterday overturned a ruling of the U.S. Parole Commission in a decision that reduces the prison sentence of an associate of former Maryland Gov. Marvin Mandel and raises the possibility that Mandel himself might win a similar reduction.

The ruling, which rejects the parole commission's application of its own guidelines, cuts at least four months off the sentence of one of Mandel's codefendants, Harry W. Rodgers III. Mandel's attorney, Arnold Weiner, said last night he expects to use the ruling to win an earlier release date for the former governor, who is not presently expected to be free until next May.

It was not immediately clear to what extent the ruling by U.S. magistrate Robert L. Crongeyer Jr. would apply to Mandel.

Although Weiner said the same considerations would apply to Mandel as to Rodgers, Nick Geeker, the federal prosecutor in northern Florida, where both men are held in a federal prison camp at Eglin Air Force Base, said the Mandel case "may be governed by different standards."

Yesterday's ruling came in response to the latest in a series of legal challenges to the actions of the parole commission filed by attorney Thomas C. Green in an effort to advance the January 1982 release date set for Rodgers. The commission has great power in determining the length of prisoners' sentences.

Rodgers, convicted with Mandel and four others of political corruption charges in August 1977, entered Eglin, a minimum security prison, in May 1980.

Based on current guidelines calling for sentences of 24 to 36 months for offenses such as those of which Rodgers was convicted, the parole commission, apparently taking into consideration evidence submitted in Rodgers' behalf, ruled that he should be freed after 20 months.

Green challenged the ruling, contending that the commission should use guidelines laid down in 1974, which were in force at the time the offenses were committed and would have dictated an earlier release date.

In June, Crongeyer ordered that the 1974 guidelines be used. However, the parole commission gave Rodgers the same release date.

In response to a new complaint by Rodgers' attorney, Crongeyer issued yesterday's ruling, which calls the commission's action "flagrant, unwarranted and/or unauthorized" and asserts that the guidelines applicable to Rodgers' case call for imprisonment of between 12 and 16 months, according to the magistrate's legal assistant, Ralph Peterson.

The magistrate also ruled that the parole commission, consistent with previous decisions, could not require Rodgers to serve more than the time specified in the upper limit of the guidelines, 16 months.

The commission could also release him sooner. Since Rodgers has already served 15 months, the ruling, if not appealed, means that the self-made insurance executive could not be required to serve more than another month at Eglin.

It could not be learned last night whether an effort will be made to overturn the ruling.

Rodgers, Mandel and their codefendants were convicted in Baltimore by a federal court jury that was told that the governor had accepted lavish gifts from his friends in exchange for his help on legislation benefiting them in connection with their secret purchase of the rundown Marlboro Race Track in Prince George's County.

The 24-to-36-month guidelines originally followed by the parole commission were set for "fraud-type property offenses," according to Rodgers' attorney. In later deliberations, they apparently used a 26-to-36-month guideline applicable to certain aggravated offenses, according to a source familiar with the case.

According to the judge's ruling, the applicable guideline was the one calling for 12 to 16 months in cases involving the bribery of a public official.

Mandel's attorney, Weiner, last night called Mandel's case identical to that of Rodgers, and said the ruling in Rodgers' favor "ought to have a direct and immediate effect on the Mandel case." He said "we intend to pursue the same remedy. I would expect the outcome should be the same."

Rodgers' attorney, Green, also expressed the view that Mandel's attorneys "are in a position to invoke the same arguments and principles and secure for him a release earlier than that now set..."

However, one source familiar with the ruling in the Rodgers case said that the discretion available to the parole commission makes it too early to predict the outcome of an appeal on behalf of the former governor.